I doubt I can explain it well, but this particular blockage was caused by needing to learn and write too much for the particular topic. The breakthrough was to find ways to cut the material into bite-size pieces.
I trust you appreciated the two metaphors thrown into the above paragraphs: swimming or eating. Note how I carefully avoided eating just before swimming.
Writing Tip: Break large tasks into manageable chunks.
Tip for Web-Based Copy-and-Paste:
Copy from a "rich" formatted client into a pure text file, like Notepad; then copy from Notepad into the next web-based editor.
Otherwise the formatting might carry forward from one to the other, leaving you with unwanted heading, font or colour style features.